Over recent years there have been some catastrophic earthquakes around the world. Therefore, it is not a surprise that Singaporean authorities are moving towards seismic design for critical structures such as Hospitals. However, it is important to highlight that designing the structures to withstand seismic actions is only part of the puzzle. In addition to the seismic actions, it is vital that the detailing of the structure, specification of the materials and selection of the products is also taken in to consideration.
In fact, some seismic tests for products are quasi static in nature and do not truly represent an actual earthquake. Therefore, additional tests such as Charpy V-Notch impact tests might also be needed to prove fitness for use in a seismic region.
In the case of a façade anchoring, where a failure can easily see a 200+kg panel falling from the side of a high rise structure, this suitability is especially critical as the current method of anchoring them to the structure usually uses cast in channels that only have high load carrying capacity in 2 axes.
With large lateral ground accelerations during an earthquake, the applied loads in the third axis (along the length of the channel) are much higher when seismic actions are considered. This has led to the development of serrated or toothed channels which have a significantly higher capacity in the third axis.
Anchor Channels are regulated in Europe under the European Assessment Document (EAD) 330008-02-06014 and designed with the standard PrEN 1992-4. In the United States of America they are tested to AC232 – Acceptance Criteria for anchor channels in Concrete5 by the Evaluation Service of the International Code Council (ICC-ES), but currently in Singapore we do not have a certification process and tend to follow the European Technical Assessments (ETAs). These documents all use the same design principles for tension and shear loads perpendicular to the longitudinal axes of the anchor channel. Since February 2016, AC232 also covers loads in the longitudinal axis of the anchor channel and includes the design for seismic conditions allowing their use in all seismic design categories (A – F) provided the channels are capable of providing resistance in all three axes. A common approach to achieve a load capacity in the longitudinal direction is to use the above mentioned serrated channels.
Serrated channels contain teeth along the inside of the channel lips and are used with channel bolts with a matching serration. The channel and bolt serrations create an interlock between the two surfaces which provides a positive connection capable of resisting shear loads in the longitudinal direction of the channel axis. Toothed Anchor Channels have been used in multiple projects worldwide including the anchoring of the curtain wall façade of the Broad Museum in Los Angeles or the Tanjong Pagar Mixed Development project which required particular attention to seismic design.
Plain anchor channels with plain T-bolts can take over longitudinal loads based on the friction between the channel and the T-bolt, but the capacity relies on the torque of the T-bolt. International standards and regulations do not allow friction based connections and thus plain anchor channels in combination with plain T-bolts cannot be used in applications that require longitudinal loading.
Anchor channels with plain and toothed lips and T-bolts
The design approach as described in AC 232 allows for a design of serrated anchor channels to provide a safe and reliable way to connect façade panels to concrete. The use of the slotted holes will still allow the adjustability required to overcome building tolerances and the absence of welding will increase safety and productivity.